1. Field of the Invention
The present invention relates to a focus detecting device employing a so-called passive auto-focus (xe2x80x9cAFxe2x80x9d) system (system for detecting a focus by light from a subject) such as a phase detection system, a contrast detecting system, and the like mounted on a photographing apparatus such as a camera, video camera, and the like and on various observing apparatuses, and more particularly, to a focus detecting device permitting auxiliary light to be projected effectively in the passive AF system.
2. Description of the Related Art
FIG. 15 shows an example of a camera to which a conventional focus detecting device is assembled. In FIG. 15, reference numeral 100 denotes an auxiliary light emitting device, reference numeral 101 denotes an objective lens for taking a picture, reference numeral 102 denotes a semi-transparent main mirror, reference numeral 103 denotes a focusing plate, reference numeral 104 denotes a pentaprism, reference numeral 105 denotes an eyepiece, reference numeral 106 denotes a sub-mirror, reference numeral 107 denotes a film, and reference numeral 108 denotes a focus detecting device, respectively. In FIG. 15, light beams coming from a subject (not shown) are reflected by the main mirror 102 upward after they pass through the objective lens 101 and form an image on the focusing plate 103. The image formed on the focusing plate 103 is visually confirmed by a photographer or an observer through the eyepiece 105 after it is reflected by the pentaprism 104 a plurality of times.
In contrast, some light beams having reached the main mirror 102 from the objective lens 101 partly pass through the main mirror 102 and are reflected downward by the sub-mirror 106 and introduced to the focus detecting device 108.
FIG. 16 is a view in which only the objective lens 101 and the focus detecting device 108 of FIG. 15 are illustrated to explain a principle of focus detection.
In the interior of the focus detecting device 108 of FIG. 16, reference numeral 109 denotes field of view masks disposed at positions near to a scheduled focusing surface of the objective lens 101, that is, near to a surface conjugate with a film surface, reference numeral 110 denotes a field lens similarly disposed in the vicinity of the scheduled focusing surface, reference numeral 111 denotes a secondary imaging system composed of two lenses 111-1 and 111-2, reference numeral 112 denotes a photoelectric conversion element including two sensor rows 112-1 and 112-1 and disposed in correspondence to the two lenses 111-1 and 111-2 there behind, reference numeral 113 denotes a diaphragm including two openings 113-1 and 113-2 disposed in correspondence to the two lenses 111-1 and 111-2, and reference numeral 114 denotes a projecting pupil of the objective lens 101 including two divided regions 114-1 and 114-2, respectively. Note that the field lens 110 has an action for imaging the openings 113-1 and 113-2 of the diaphragm 113 in the vicinity of the regions 114-1 and 114-2 of the projecting pupil 114 of the objective lens 101, so that light beams 115-1 and 115-2 having passed through the respective regions 114-1 and 114-2 form distributed quantities of light in the two sensor rows 112-1 and 112-2, respectively.
The focus detecting device 108 employs a so-called phase difference detecting system arranged such that when an imaging point of the objective lens 101 is located forward of the scheduled focusing surface, that is, on the objective lens 101 side, the distributed quantities of light formed on the two sensor rows 112-1 and 112-2, respectively, are separated from each other. Moreover, an amount of deviation of the distributed quantities of light formed on the two sensor rows 112-1 and 112-2 has a certain functional relationship with an amount of defocus of the objective lens 101, that is, with an amount out of focus. Thus, when the amount of deviation is calculated by an appropriate calculation unit, an out of focus direction and amount of the objective lens 101 can be detected.
Further, to cope with conditions under which a focus cannot be detected accurately or cannot be detected at all because a subject is dark or has a low contrast, a camera having an auxiliary light emitting device mounted thereon or built therein is proposed to illuminate the subject or to project a pattern having a contrast onto the subject.
FIG. 17 is a view showing an arrangement of an ordinary auxiliary light emitting device, wherein light from a light emission unit 116 illuminates a pattern 117 and a light projection lens 118 for projecting the pattern 117 onto a subject is protected by a protection plate 119.
FIG. 18 shows a pattern image 120 formed on the subject by the pattern 117.
A camera having a focus detecting device assembled thereto cannot detect a focus when a subject has a low degree of luminance. Thus, detection of a focus is made possible in a subject having a low degree of luminance by increasing a contrast of the subject by illuminating the subject or projecting a pattern thereto by an auxiliary light emitting device. However, a focus detecting capability in a low degree of luminance greatly depends upon determination conditions for determining whether or not auxiliary light is to be emitted because auxiliary light (i) may be wastefully projected without any sufficient effect, or (ii) may not be emitted regardless of whether an increase in a focus detecting accuracy could be expected, depending upon a reflectance of a subject, a state of a background, and a focus detecting capability of a focus detecting device.
Recently, the number of focus detecting points has been increased and a focus detecting region has been enlarged. Accordingly, when a determination criterion for determining whether or not auxiliary light is to be projected is uniformly set in a focus detecting device having a plurality of focus detecting points, a problem arises in that wasteful projection of auxiliary light is more and more increased and that no auxiliary light is projected regardless of whether an increase of a focus detecting accuracy could be expected thereby because respective focus detecting points are located at different positions on a subject and focus detecting capabilities are different at the respective focus detecting points.
The present invention, in one aspect, is directed to an auxiliary light emitting device for a focus detecting device for emitting an auxiliary light, the focus detecting device having, a screen divided into a plurality of regions and a plurality of focus detecting areas respectively disposed on the plurality of regions, the focus detecting device respectively detecting a focus of a subject in the plurality of focus detecting areas. The auxiliary light emitting device comprises a determination circuit for determining whether the auxiliary light is to be emitted based on a determination reference value, and setting means for setting the determination reference value to a predetermined value according to a position of at least one of the plurality of focus detecting areas.
In another aspect, the present invention is directed to an auxiliary light emitting device for a focus detecting device for emitting an auxiliary light, the focus detecting device having a screen divided into a plurality of regions and a plurality of focus detecting areas respectively disposed on the plurality of regions, the focus detecting device respectively detecting a focus of a subject in the plurality of focus detecting areas. The auxiliary light emitting device comprises a selection circuit for selecting at least one of the plurality of focus detecting areas, and a setting circuit for setting an auxiliary light emission determination value to a predetermined value in accordance with the at least one focus detecting area selected by the selection circuit, the auxiliary light emission determination value being a criterion for determining whether the auxiliary light is to be emitted.
In yet another aspect, the present invention is directed to a focus detecting device. The focus detecting device comprises a plurality of focus detecting areas; a selection circuit having modes including (i) an optional selection mode for selecting an optional focus detecting area, and (ii) an automatic selection mode for automatically selecting at least one of the plurality of focus detecting areas; a setting circuit for setting, (i) in the optional selection mode, determination criteria to values according to the plurality of focus detecting areas, respectively, and a selected determination criterion to the determination criterion value for the focus detecting area selected by the selection circuit, and (ii) in the automatic selection mode, the selected determination criterion to a lowest of the determination criteria values in the optional selection mode; and a determination circuit for determining whether auxiliary light is to be emitted based on the selected determination criterion.
In yet another aspect, the present invention is directed to a focus detecting device having an auxiliary light mode, in which, when a focus is not appropriately first detected, a focus detecting operation for detecting the focus is executed with auxiliary light being projected. The focus detecting device comprises a plurality of focus detecting areas; detecting means for detecting a focus in each of the plurality of focus detecting areas; and a determination circuit for determining whether the focus detecting operation is to be executed in the auxiliary light mode based on the detected focus and a determination criterion, wherein the determination criterion is set to a predetermined value according to at least one of the plurality of focus detecting areas.
In yet another aspect, the present invention is directed to a camera. The camera comprises a focus detecting device including a plurality of focus detecting areas, the focus detecting device detecting a focus in each of the plurality of focus detecting areas, and having a first selection mode for selecting at least one of the plurality of focus detecting areas by manual selection of an operator and a second selection mode for automatically selecting at least one of the plurality of focus detecting areas; a setting circuit for setting, (i) in the first selection mode, a selected determination criterion to a determination criterion value for a selected focus detecting area, and (ii) in the second selection mode, the selected determination criterion to a fixed criterion; and auxiliary light mode setting means for determining, based on the selected determination criterion, whether to execute a focus detecting operation with auxiliary light being projected, and for executing the focus detecting operation with the auxiliary light being projected.
Further objects, features, and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments with reference to the attached drawings.